1. Field of the Invention
The present invention relates to a driving-force distribution control device for distributing a driving force properly to individual drive wheels of a vehicle.
2. Description of the Related Art
Drive wheels of an automobile is provided with a differential gear (a differential device) between the left wheels and the right wheels. As a result, the automobile can turn smoothly while allowing a difference in the rotating speed between the left-right wheels at the turning time. Generally in the case of a four-wheel drive vehicle, moreover, not only the aforementioned left-right wheel differential gear but also a differential gear (a center-diff) is disposed between the front wheels and the rear wheels so that the rotating speed difference between the front and rear wheels can be allowed.
In recent years, vehicles imaging to be used on rough roads, and vehicles imaging to be used for sporty driving are frequently provided with a differential limiting device for limiting the differential motion due to the differential gear. This differential limiting device is represented by a limited slip differential (LSD), which can be electrically or mechanically controlled to distribute a proper driving force to the desired drive wheels even if any drive wheel slips.
Further, there has been developed a driving force distributing system, which is intended not only for the case of a mere slip of drive wheels, but also positively activates the aforementioned LSD to improve the turning performances, the accelerating performances and the stability performances of the vehicle. This driving force distributing system is exemplified by a front-rear wheel driving-force distributing mechanism capable of changing the driving force distribution of the front-rear wheels, a left-right wheel driving force distributing mechanism capable of changing the driving force distribution of the left-right wheels, an electronic control LSD, an electronic control coupling or the like.
On the other hand, a vehicle equipped with an anti-skid brake system (ABS) has been developed and put into use so that the vehicle can be reliably decelerated while retaining its steerability during breaking.
Here, the vehicle having both the aforementioned driving-force distributing system and the ABS have a problem that the control by the driving-force distributing mechanism and the control by the ABS interfere with one another. For example, Japanese Patent Publication No. 2005-104233A discloses a technology that can solve that problem and improve the control affinity between the driving-force distributing system and the ABS. According to this technique of Japanese Patent Publication No. 2005-104233A, the control mode of the driving-force distributing system is suitably interchanged in response to the action/inaction of the ABS between an ordinary mode and a control mode (an ABS corresponding mode) that does not obstruct the effect by the ABS control, and in the ABS corresponding mode, switching depending on whether the road surface μ is a high μ (corresponding to a dry asphalt road surface) or a low μ (corresponding to a wet asphalt road surface). On the high-μ road, more specifically, the control by the left-right wheel driving-force distributing mechanism is given preference, but the control by the ABS is given preference for the longitudinal direction. On the low-μ road, the control mode is changed so that the entire control by the ABS is given preference. Moreover, the specific control modes of the driving-force distribution control are exemplified by the rotating speed difference control (a ΔN control) between the front-rear wheels and the left-right wheels, the acceleration corresponding control, the deceleration corresponding control and so on. On the high-μ road, the control is made to reduce the controlled variable only for the ΔN control. On the low-μ road, the controlled variable of the ΔN control is further reduced so that only the ΔN control is made without the acceleration corresponding control or the deceleration corresponding control.
In the aforementioned technique disclosed in Japanese Patent Publication No. 2005-104233A, the control mode of the driving-force distributing system during operation of the ABS basically switches two ABS corresponding modes (for the high-μ road and the low-μ road). Therefore, the technique raises a problem that the driving force cannot be finely distributed on an intermediate μ road (e.g., a wet asphalt road surface or the unpaved road), on a snow road of the road, or on an extremely low-μ road (e.g., a frozen road or on ice).
In case the road surface μ is near the switching threshold value of the two ABS corresponding modes, moreover, there arises another problem such a hunting or control under different modes despite the vehicle is running on the same a sudden change in the control from a normal run on a common road surface may occur and results in unstable control.
Among the various control modes of the driving-force distribution control, moreover, the acceleration corresponding control or the deceleration corresponding control, other than the ΔN control, belongs to either the controlled variable reducing or ON-OFF switching operation like that of the ΔN control. As a result, the fine reduction control matching the characteristics (i.e., the interfering degree to the control by the ABS) cannot be made and may raise a problem in the interference with the control by the ABS or the reduction in the control performance (e.g., the turning performances).